The present invention relates to ring laser gyroscopes and has particular relation to methods for calibrating the scale factor for same.
A ring laser gyroscope or ring laser gyro (RLG) is suppose to determine through what angle the apparatus to which it is attached, such as an intercontinental ballistic missile (ICBM). has rotated. What it actually determines is how many interference fringes it has detected. A scale factor must be calculated which, when multiplied by the number of fringes, gives the angle of rotation. This scale factor is unique for each ring laser gyro, and is calculated empirically by rotating the gyro through a known angle and counting the number of fringes which are detected. It may also be calculated by rotating the RLG at a known rate of rotation and measuring the fringe rate (number of fringes per second). Measuring the "scale factor," therefore, as that term is used herein, is synonymous with measuring the angle, number of fringes, rate of rotation, or fringe rate, as appropriate for the context, and these terms are synonymous with one another.
Unfortunately, an RLG's scale factor changes with time. It is also subject to being changed by the radiation pulse produced by the nearby detonation of a nuclear device. It is therefore important that an RLG in an ICBM be able to be calibrated after the ICBM has survived a first strike Needless to say, it is impractical, at that time, to remove the RLG from the missile, factory calibrate it, and replace it in the missile before a launch.
In general, the scale factor is a function of the intensity of the beam, which may be measured by a suitable intensity monitor. While a stable intensity is desirable, the actual intensity (after aging, radiation pulse, or the like) is all that is required to calculate the actual scale factor. Accordingly, the output voltage from the intensity monitor is fed to a suitable scale factor calculator, and the appropriate compensation is made. Unfortunately, the same aging or radiation pulse which changes the scale factor will also affect the intensity monitor. While the monitor will still produce an output voltage which is proportional to intensity, the proportionality constant (volts of output signal per watt of beam intensity) will be changed by the pulse. A false compensation will therefore be provided. This invention provides an approach to an in-flight calibration of the intensity monitor so that the correct compensation can be made to the scale factor.
A calibration opportunity exists between the end of the boost phase and the beginning of the reentry phase. During this post-boost phase, the missile is spinning rapidly, much like a bullet, but is too far above the earth's surface to be intercepted or even effectively interfered with. The conventional method for calibrating the RLG during this post-boost phase is to determine the actual rotation angle by sensing the sun, a star, or a radio source on the earth, and to compare the actual angle with the calculated angle. The scale factor may be determined from this comparison. The present invention avoids such external input, and calculates the scale factor in an entirely self-contained fashion by first calculating the correct calibration coefficient for the intensity monitor and then applying the corrected output voltage from the intensity monitor to the scale factor calculator.